You might encounter some issues when installing Microsoft Security Essentials. This is usually because you have some other security program running or because you're missing some prerequisites. To help with these issues, follow the instructions that follow. You’ll need to restart your computer, so please print this page or bookmark it.

Note

If you can’t install Microsoft Security Essentials because you can’t even download it, the first thing to try is your Internet connection (but if you’re viewing this page, that’s probably fine). You may also want to try downloading from Windows Internet Explorer.